The aim of this study was to investigate the effect of season, temperature, humidity, age of the boar, and semen collection interval on sperm morphology in Duroc boars in Thailand, kept either in a conventional open air system (CONV) or in an evaporative cooling system (EVAP). In total, 1176 ejaculates from 110 sexually mature boars in six CONV herds and five EVAP herds were morphologically examined during a one-year period. Analysis of variance was applied to the data. Minor differences in the sperm morphology traits analyzed were found between the housing systems. There was a significant seasonal effect (two-month periods) on the percentage of morphologically normal spermatozoa (normal1), morphologically normal spermatozoa including spermatozoa with distal cytoplasmic droplets (normal2), proximal cytoplasmic droplets (prox), and sperm head abnormalities (P<or=0.001). Temperature had a significant effect on normal1 in the CONV system (P<or=0.01) and in the EVAP system (P<or=0.05), and on normal2 (P<or=0.05) and prox (P<or=0.001) in both systems. Humidity had a significant effect on prox (P<or=0.05) in the EVAP system. The results also indicate that the sperm morphology traits were mildly affected by the housing systems investigated. A seasonal variation in sperm morphology was found, and high temperatures and high humidity had negative effects on sperm morphology.